Low molecular weight α-hydroxy carboxylic and dicarboxylic acids in reducing marine sediments

The occurrence and distribution of low molecular weight α-hydroxy carboxylic and dicarboxylic acids was studied in reducing marine sediments collected in the Santa Barbara Basin and the Cariaco Trench. Four compounds were found to occur in both basin sediments: glycolic, lactic, oxalic and succinic acids. In general concentrations were low (? 1 μmol/g for the hydroxy acids and ? 100μmol/g for the dicarboxylic acids), and generally decreased with depth. Subsurface maxima were observed for lactate and succinate in the Santa Barbara Basin. Both the vertical profiles and lactate enantiomer ratios suggested microbial origin and control for the distribution of these compounds. A preliminary model for the microbial mediation of the early diagenesis of organic compounds in reducing marine sediments is presented.     

Changes of lead and barium with time in California off-shore basin sediments

During ancient times the natural deposition fluxes of lead which can be leached with dilute acid from sediments in Santa Barbara, Santa Monica and San Pedro basins offshore from the Los Angeles Urban complex, were about 0.7, 0.1 and 0.2 μg Pb/cm² yr respectively. Since there was little difference in biological productivity in surface waters of these basins, it is proposed that clay is a major transport vehicle for sequestered soluble lead, which then explains why the lead deposition flux within the Santa Barbara basin was so much larger compared to the other basins. The fluxes of silicate mud in the basins in ancient times were about 92, 19 and 30 mg/cm² yr in Santa Barbara, Santa Monica and San Pedro basins respectively. Today deposition fluxes of acid soluble lead within these three basins are 3- to 9-fold greater, being about 2.1, 1.1 and 1.8 μg Pb/cm² yr respectively, partly in the form of directly deposited large sewage particles, which account for none, $\text{2}\text{3}$ and $\text{3}\text{4}$ of the total industrial lead deposition fluxes in the respective basins. Concentrations of leachable lead in varve dated sediment layers increase with time and isotopic compositions of these leads change in accordance with corresponding known changes of isotopic compositions of industrial lead in the Los Angeles atmosphere. Lead remaining in acid leached sediment residues originates from igneous and clay minerals, exhibiting no change in concentration or isotopic composition since pre-industrial times.Deposition fluxes of total barium in sediments among the three basins were proportional to mass deposition fluxes before 1950 in the same manner as for lead. Afterwards, there are barium concentration maxima with time in both Santa Monica and San Pedro Basin sediments which are attributable to industrial sewage rather than to episodic erosion from barium-rich sedimentary evaporite strata exposed locally along the shore. An increase of barium concentrations in present day Santa Barbara basin sediments may reflect dispersal of barium-rich drilling mud from local drilling operations.     

Tropical forcing of North Pacific intermediate water distribution during Late Quaternary rapid climate change?

Evidence is presented demonstrating intermediate water (∼500 m) temperature variability at ODP Hole 893A in Santa Barbara Basin during submillennial climate change (11–60 ka). Benthic δ¹⁸O oscillations are considered to result primarily from shifts in intermediate water temperature at the site. Detailed comparison of both benthic and planktonic records from the basin provide crucial evidence for differing surface and intermediate water mass temporal responses to rapid climate change. Gradual warming of intermediate water compared to abrupt cooling suggests mechanistic differences between processes controlling North Pacific Intermediate Water expansion and contraction relative to ‘southern component’ intermediate waters. Comparisons suggest intermediate water warming preceded (by 60–200 years) the most rapid interval of surface warming inferred to be associated with North Pacific atmospheric reorganization. Tropical forcing of sea level anomalies in the eastern Pacific via trade wind strength may control California Undercurrent flow (300–500 m) and be the cause of early intermediate water warming in Santa Barbara Basin.     

Paleoceanography of the Santa Barbara Basin during the last 8000 years

The varved sediments of the Santa Barbara Basin off southern California, offer a unique opportunity to study the changes in oceanographic conditions of this nearshore area during the last 8000 yr. Quantitative analysis of Radiolaria found in recent surface sediment samples from the eastern North Pacific allows the identification of four “assemblages” which can be related to the physical oceanography of the California Current. Two assemblages are associated with the southerly flowing California Current, one with the main stream of the current (California Current Assemblage) and the other with the offshore flow along northern California (Central Assemblage). The two other assemblages are associated with the subtropical region of the eastern North Pacific (Subtropical Assemblage) and one found mostly off the coast of Baja California (Baja Assemblage). Analysis of the Radiolaria found in the varved sediments of a core from the Santa Barbara Basin give an 8000-yr continuous record of these four assemblages. The California and Baja Assemblages show only minor fluctuations in their importance in the sediments of the Santa Barbara Basin. The California Assemblage, however, shows a steady increase during this time period. Prior to 5400 yr B.P. the Radiolaria were predominately subtropical in character, whereas after 5400 yr B.P. the Central Assemblage becomes more important. Since 5400 yr B.P. most of the changes in the radiolarian fauna consist of fluctuations in the importance of these two assemblages. Past sea-surface temperatures for the month of February were calculated using the transfer function technique of J. Imbrie and N. G. Kipp (1971, In “The Late Cenozore Glacial Ages” L. K. Turekian, Ed.), (Chap. 5, Yale Univ. Press, New Haven, Conn.). The time series of paleotemperature estimates show major changes in the average February temperature of Santa Barbara Basin waters. The range of estimated temperatures (12°C) exceeds that of the historical observations of February temperatures in the Santa Barbara Basin but does not exceed the observed range for the California Current region. The intervals from 800 to 1800 yr B.P. 3600 to 3800 yr B.P. and 5400 to the end of the record appear to have been generally warmer than today. Comparison of the Holocene record of alpine glacial advances with the radiolarian assemblage and paleotemperature time series shows that the initiations of advances was coincident with a decrease in sea-surface temperatures and an increase in the importance of the Central Assemblage in the Santa Barbara Basin. The terminations of these advances were not marked by any consistent characteristic in the Santa Barbara Basin time series.     

Preservation of particulate non-lithogenic uranium in marine sediments

Particulate non-lithogenic uranium (PNU), excess U above detrital background levels found in marine particulate matter, is formed in surface waters throughout the ocean. Previous studies have shown that PNU is regenerated completely prior to burial of particles in sediments within well-oxygenated open-ocean regions. However, the fate of PNU has never been examined in ocean margin regions or in anoxic basins. Here we evaluate the preservation of PNU in ocean margin sediments and within semi-enclosed basins using samples from sediment traps deployed at multiple depths and surface sediments. Organic carbon fluxes at the sediment trap locations ranged from 0.1 to 4.3 g/cm² kyr, while the dissolved oxygen concentration in the water column ranged from <3 μM to ∼ 270 μM. Preservation of PNU increases with decreasing dissolved oxygen concentration, approaching 100% preservation at oxygen concentration < 25 μM. PNU contributes as much as 40 to 70% of the total authigenic U in sediments in the Santa Barbara Basin and seasonally anoxic Saanich Inlet, and some 10% to 50% of the total authigenic U in sediments off the central California Margin.     

Geological and anthropogenic influences on sediment metal composition in the upper Paracatu River Basin, Brazil

This work reports a geochemical study of sediments from the upper Paracatu River Basin. The objective is to define the influences of Au, Zn, and Pb mineral deposits and mining activities on the sediment metal sources, distribution, and accretion. The samples were analyzed using ICP/OES, AAS, and XRD techniques and were treated with principal components analysis and the geo-accumulation index. The main geochemical processes that control the sediment composition are pyrite oxidation, muscovite weathering, carbonate dissolution, and the erosion of oxisols enriched with Zn and Pb. The upper Rico Stream has high Al, Fe, Cu, Cr, Co, and Mn concentrations due erosion of oxisols and pyrite oxidation and muscovite alteration present in the parental rock. The artisanal alluvial gold mining increased the primary rock-minerals?? weathering and Hg sediment concentration. The lower Escuro River and Santa Catarina Stream are enriched with Zn and Pb due the erosion of metal-rich soils formed over galena, sphalerite, calamine, and willemite mineral deposits located upstream. Elements such as Ca, Mg, and Ba have low concentrations throughout the sampled area due the high solubility of these metals-bearing minerals. The dispersion of metals is limited by the basin geomorphology and their affinity to silt-clayey minerals and Fe and Mn oxides and hydroxides in circumneutral pH waters.     

Geochemical fractionation of Ni,Cu and Pb in the deep sea sediments from the Central Indian Ocean Basin: An insight into the mechanism of metal enrichment in sediment

Metal speciation study in combination with major element chemistry of deep sea sediments provided possible metal enrichment pathways in sediments collected from environmentally different locations of Central Indian Ocean Basin (CIB). Metal speciation study suggests that Fe–Mn oxyhydroxide phase was the major binding phase for Ni, Cu and Pb in the sediments. The second highest concentrations of all these metals were present within the structure of the sediments. Easily reducible oxide phase (within the Fe–Mn oxyhydroxide binding phases) was the major host for all the three metals in the studied sediments. Major element chemistry of these sediments revealed that there was an increased tendency of Cu and Ni to get incorporated into the deep sea sediment via the non-terrigenous Mn-oxyhydroxide fraction, whereas, Pb gets incorporated mostly via amorphous Fe-hydroxides into the sediment from the CIB. This is the first attempt to provide an insight into the mechanism of metal enrichment in sediment that host vast manganese nodule.     

Manganese metalliferous sediments of the Deryugin Basin (Sea of Okhotsk): Chemical composition and geological setting

This paper reports the results of an investigation of the chemical composition and distribution of manganese-rich metalliferous sediments in the Deryugin Basin. They compose an upper sedimentary layer up to 30 cm thick within a strip up to 40 km wide and 180 km long in the central part of the basin along the East Deryugin suture zone bounding the Deryugin sedimentary basin to the east. The sediments are enriched relative to the regional background composition in Mn (by a factor of 35); Au; Ni; Zn; Co (by a factor of 3–5); and, locally, in Mo, V, Ba, Cu, Pb, and Hg. It was supposed that their formation was controlled by several processes: precipitation of Mn and some other trace elements from hydrothermal plumes of seawater that occasionally form above submarine hot vents in the northern part of the basin, precipitation from anoxic bottom waters at the eastern boundary of the Deryugin sedimentary basin, and early diagenetic migration of Mn into the surface sediments.     

The contrasting biogeochemistry of iron and manganese in the Pacific Ocean

Vertical and horizontal distributions of dissolved and suspended particulate Fe and Mn, and vertical fluxes of these metals (obtained with sediment traps) were determined throughout the Pacific Ocean. In general, dissolved Fe is low in surface and deep waters (0.1 to 0.7 nmol/kg), with maxima associated with the intermediate depth oxygen minimum zone (2.0 to 6.6 nmol/kg). Vertical distributions of dissolved Mn are similar to previous reports, exhibiting a surface maximum, a subsurface minimum, a Mn maximum layer coincident with the oxygen minimum zone, and lowest values in deep waters.Near-shore removal processes are more intense for dissolved Fe than for dissolved Mn. Dissolved Mn in the surface mixed layer remains elevated much farther offshore than dissolved Fe. Elevated near-surface dissolved Mn concentrations occur in the North Pacific Equatorial Current, suggesting transport from the eastern boundary. Near-surface mixed-layer dissolved Mn concentrations are higher in the North Pacific gyre reflecting enhanced northern hemisphere aeolian sources.Residence time estimates for the settling of refractory paniculate Fe and Mn from the upper water column are 62–220 days (Fe), and 105–235 days (Mn). In contrast, residence times for the scavenging of dissolved Fe and Mn are 2–13 years (Fe) and 3–74 years (Mn). Scavenging residence times for dissolved Mn based on horizontal mixing in the surface mixed layer of the northeast Pacific are 0.4 years (nearshore) to 19 years (1000 km offshore).There is no evidence for in situ Fe redox dissolution within sub-oxic waters in the eastern tropical North Pacific. Dissolved Fe appeared to be controlled by dissolution from sub-oxic sediments, with oxidative scavenging in the water column or upper sediment layers. However, in situ Mn dissolution within the oxygen minimum zone was evident.     

Diagenesis of oxyanions (V, U, Re, and Mo) in pore waters and sediments from a continental margin

This research tests the hypothesis that trace metals respond to the extent of reducing conditions in a predictable way. We describe pore water and sediment measurements of iron (Fe), manganese (Mn), vanadium (V), uranium (U), rhenium (Re), and molybdenum (Mo) along a transect off Washington State (USA). Sediments become less reducing away from the continent, and the stations have a range of oxygen penetration depths (depth to unmeasurable O₂ concentration) varying from a few millimeters to five centimeters. When oxygen penetrates ∼1 cm or less, Fe is reduced in the pore waters but reoxidized near the sediment-water interface, preventing a flux of Fe²⁺ to overlying waters, whereas Mn oxides are reduced and Mn²⁺ diffuses to overlying waters. Both Re and U authigenically accumulate in sediments. Only at the most reducing location, where the oxygen penetrates 0.3 cm below the sediment-water interface, does the surface 30 cm of sediments become reducing enough to authigenically accumulate Mo.Stations in close proximity to the Juan de Fuca Ridge crest are enriched in Mn and Fe from hydrothermal plume processes. Both V and Mo clearly associate with Mn cycling, whereas U may be associating with either Mn oxides and/or Fe oxyhydroxides. Rhenium is uncomplicated by adsorption to Mn oxides and/or Fe oxyhydroxides, and Re accumulation in sediments appears to be due solely to the extent of reducing conditions. Therefore, authigenic sediment Re enrichment appears to be the best indicator for intermediate reducing conditions, where oxygen penetrates less than ∼1 cm below the sediment-water interface, when coupled with negligible authigenic Mo enrichment.     

Spatial geochemical variation of major and trace elements in the marine sediments of Mumbai Harbor Bay

The resulting concentration data sets of major (Na, K, Mg, Ca and Fe) and trace elements (Cu, Ni, Co and Mn) in bed and suspended sediments were used to evaluate the enrichment factor for anthropogenic influences and principal component analysis for identifying the origin of source contributions in the studied area. Normalization of metals to Fe indicated that high enrichment factors in the bed sediment were in the order of Co > Cu > Na > Ca > Ni except Mg, K and Mn while for suspended sediments, only Co has a high enrichment factor. High enrichment of Co and Cu reflected the contamination of sediments from anthropogenic sources. The high influence of Na and Ca in sediments may be caused for seawater salinization factor. A significant positive correlation among enrichment factors of various elements of interest suggests a common origin/identical behavior during transport in the sediment system.     

Uranium transport in the Walker River Basin,California and Nevada

During the summer of 1976 waters from tributaries, rivers, springs and wells were sampled in the Walker River Basin. Snow and sediments from selected sites were also sampled. All samples were analyzed for uranium and other elements. The resulting data provide an understanding of the transport of uranium within a closed hydrologic basin as well as providing a basis for the design of geochemical reconnaissance studies for the Basin and Range Province of the Western United States.Spring and tributary data are useful in locating areas containing anomalous concentrations of uranium. However, agricultural practices obscure the presence of known uranium deposits and render impossible the detection of other known deposits.Uranium is extremely mobile in stream waters and does not appear to sorb or precipitate. Uranium has a long residence time (2500 years) in the open waters of Walker Lake; however, once it crosses the sediment-water interface, it is reduced to the U(IV) state and is lost from solution.Over the past two million years the amount of uranium transported to the terminal point of the Walker River system may have been on the order of 4 × 10⁸kg. This suggests that closed basin termini are sites for significant uranium accumulations and are, therefore, potential sites of uranium ore deposits.     

A 560-Year Record of Santa Ana Fires Reconstructed from Charcoal Deposited in the Santa Barbara Basin, California

Microscopic charcoal from varved Santa Barbara Basin sediments was used to reconstruct a 560-yr record (A.D. 1425 to 1985) of Santa Ana fires. Comparison of large (>3750 μm²) charcoal with documented fire records in the Santa Barbara Ranger District shows that high accumulations correspond to large fires (>20,000 ha) that occurred during Santa Ana conditions. The charcoal record reconstructed a minimum of 20 large fires in the Santa Barbara region during the study period. The average time between fires shows no distinct change across three different land use periods: the Chumash period, apparently characterized by frequent burning, the Spanish/Early American period with nominal fire control, and the 20th century with active fire suppression. Pollen data support the conclusion that the fire regime has not dramatically changed during the last 500 yr. Comparison of large charcoal particle accumulation rates and precipitation reconstructed from tree rings show a strong relationship between climate and fire history, with large fires consistently occurring at the end of wet periods and the beginning of droughts.     

Geochemistry and genetic model of the ore-bearing sediments of the Parnok ferromanganese deposit, Polar Urals

The Parnok ferromanganese deposit is confined to the black shales of the western slope of the Polar Urals. The deposit area is made up of weakly metamorphosed terrigenous-carbonate rocks formed in a marine basin at a passive continental margin. Ore-bearing sequence is composed of coaliferous clayey-siliceous-calcareous shales comprising beds and lenses of pelitomorphic limestones, and iron and manganese ores. The iron ores practically completely consist of micrograined massive magnetite. The manganese ores are represented by lenticular-bedded rocks consisting of hausmannite, rhodochrosite, and diverse manganese silicates. With respect to relations between indicator elements (Fe, Mn, Al, Ti), the shales are ascribed to pelagic sediments with normal concentrations of Fe and Mn, the limestones correspond to metalliferous sediments, ferruginous sediments are ore-bearing sediments, while manganese rocks occupy an intermediate position. It was found that the concentrations of trace elements typical of submarine hydrothermal solutions (As, Ge, Ni, Pb, Sb, Zn, etc.) in both the ore types are in excess of those in lithogenic component. At the same time, the indicator elements of terrigenous material (Al, Ti, Hf, Nb, Th, Zr, and others) in the ores are several times lower than those in the host shales (background sediments). REE distribution patterns in iron ores show the positive Eu anomaly, while those in manganese ores, the positive Ce anomaly. In general, the chemical composition of the ores indicates their formation in the hydrothermal discharge zone. The peculiar feature of the studied object is the manifestation of hydrothermal vents in sedimentary basin without evident signs of volcanic activity. Hydrothermal solutions were formed in terrigenous-carbonate sequence mainly at the expense of buried sedimentation waters. The hydrothermal system was likely activated by rejuvenation of tectonic and magmatic processes at the basement of sedimentary sequences. Solutions leached iron, manganese, and other elements from sedimentary rocks and transported them to the seafloor. Their discharge occurred in relatively closed marine basin under intermittent anaerobic conditions. Eh-pH variations led to the differentiation of Fe and Mn and accumulation of chemically contrasting ore-bearing sediments.     

Impact of sewage and mining activities on distribution of heavy metals in the water-soil-vegetation system

Several samples of groundwater and soils and plants have been collected from Sohar (Batina region, NE Oman), which is affected by various activities such as mining, agriculture, and sewage. To characterize quality of groundwater, As and Cu concentrations have been investigated in waters collected from different wells. Comparison of data with local and international standard values revealed that groundwater in Sohar region is characterized by lower concentrations in Cu and As compared with standards. In soils collected from the same area, concentrations of heavy metals have been measured in different fractions in order to investigate the mobility of such elements and risk of vulnerability in this area. A sequential extraction procedure has been applied to surface sediments to determine the partitioning of Zn, Cu, Co, Mn, Fe, Pb, Cr, Cd and Ni among (1) exchangeable and acid-soluble phases, (2) Fe–Mn oxides, (3) organic matter and sulphides and (4) resistant phases. The results showed that the mobile fraction in the sewage area accounts only for 10 % of the total concentration in sediments while in the Cu mining area, the contribution of the mobile fraction may exceed 10 %, especially for Pb, Mn, Cd, Cu and Co. Investigation of concentrations in As and Cu in plants collected from mining and sewage areas revealed an important accumulation of these elements in leaves and may explain enrichment of As in shallow groundwater relative to deep groundwater. This investigation also showed that Cu is more available in sewage area than in mining zone, as opposed to As.     

Cycling of trace metals (Mn, Fe, Mo, U, V, Cr) in deep pore waters of intertidal flat sediments

Trace metals (Mn, Fe, Mo, U, Cr, V) were studied in pore waters of an intertidal flat located in the German Wadden Sea. The study system is an example of a permeable tidal flat system where pore water exchange is affected by tidal driven pressure gradients besides diffusion. Permanently installed in situ samplers were used to extract pore waters down to 5 m depth throughout one year. The samplers were either located close to the tidal flat margin or in central parts of the tidal flat. Despite dynamic sedimentological and hydrological conditions, the general trends with depth in deep tidal flat pore waters are remarkably similar to those observed in deep sea environments. Rates of trace metal cycling must be comparably large in order to maintain the observed pore water profiles. Trace metals further show similar general trends with depth close to the margin and in central parts of the tidal flat. Seasonal sampling revealed that V and Cr vary concurrent with seasonal changes in dissolved organic carbon (DOC) concentration. This effect is most notable close to the tidal flat margin where sulphate, DOC, and nutrients vary with season down to some metres depth. Seasonal variations of Mn, Fe, Mo, and U are by contrast limited to the upper decimetres of the sediment. Their seasonal patterns depend on organic matter supply, redox stratification, and particulate matter deposited on sediment surfaces. Pore water sampling within one tidal cycle provides evidence for pore water advection in margin sediments. During low tide pore water flow towards the creekbank is generated by a hydraulic gradient suggesting that deep pore waters may be seeping out of creekbank sediments. Owing to the enrichment of specific elements like Mn in pore water compared to sea water, seeping pore waters may have an impact on the chemistry of the open water column. Mass balance calculations reveal that the impact of deep pore waters on the Mn budget in the open water column is below 4%. Mn deep pore water discharge of the whole Wadden Sea is estimated to be about 9% of the total dissolved riverine Mn input into the Southern North Sea.     

Budgets and behaviors of uranium and thorium series isotopes in Santa Monica Basin sediments

Nine natural decay-series isotopes were measured in six box cores collected from a transect across the Santa Monica Basin. The ²¹⁰Pb-derived sedimentation rate decreases from ~80 mg/cm²-yr at the slope to ~20 mg/cm²-yr in the deep central basin. Sediment mixing prevails in sites underlying oxic waters, but is subdued in the anoxic deep basin below the sill depth. Uranium contents in sediments are controlled by levels of authigenic U, which are higher in the more reduced condition in the deep basin. Most of the authigenic U results from precipitation within the sediments.The ²³²Th-²²⁸Th disequilibrium in sediments indicates that ²²⁸Ra is lost from the sediments from a depth of ~ 10 cm upward. Modelling the distribution of excess ²²⁸Th and ²³⁴Th in the surficial layers of the deep basin sediments results in a mean sediment mixing coefficient of 0.2 cm²/yr and a sedimentation rate close to that based on ²¹⁰Pb. There is no evidence of changing sedimentation rate in the central basin during the past century. Fluxes of excess ²¹⁰Pb, ²³⁰Th and ²³¹Pa to the central Santa Monica Basin sediments are much higher than what can be predicted from local supply. Advective input of open ocean waters coupled with enhanced scavenging of these reactive nuclides at the ocean margin is considered to be the primary cause.     

新疆西天山莫托沙拉铁(锰)矿床与式可布台铁矿床地质特征对比

新疆西天山富铁矿成矿带位于伊犁板块东端,火山岛弧带弧间断陷盆内.文章详述了莫托沙拉铁(锰)矿床、式可布台铁矿床的地质特征,讨论了矿床成矿机制,对矿床的共性和个性进行了对比.认为铁质来源于海相火山喷气,海解萃取,高密度流体运移,同生断裂提供热液,贮矿盆地下陷接受沉积,最终呈向斜构造.     

The anaerobic degradation of organic matter in Danish coastal sediments: iron reduction, manganese reduction, and sulfate reduction

We used a combination of porewater and solid phase analysis, as well as a series of sediment incubations, to quantify organic carbon oxidation by dissimilatory Fe reduction, Mn reduction, and sulfate reduction, in sediments from the Skagerrak (located off the northeast coast of Jutland, Denmark). In the deep portion of the basin, surface Mn enrichments reached 3.5 wt%, and Mn reduction was the only important anaerobic carbon oxidation process in the upper 10 cm of the sediment. In the less Mn-rich sediments from intermediate depths in the basin, Fe reduction ranged from somewhat less, to far more important than sulfate reduction. Most of the Mn reduction in these sediments may have been coupled to the oxidation of acid volatile sulfides (AVS), rather than to dissimilatory reduction. High rates of metal oxide reduction at all sites were driven by active recycling of both Fe and Mn, encouraged by bioturbation. Recycling was so rapid that the residence time of Fe and Mn oxides, with respect to reduction, ranged from 70-250 days. These results require that, on average, an atom of Fe or Mn is oxidized and reduced between 100-300 times before ultimate burial into the sediment. We observed that dissolved Mn2+ was completely removed onto fully oxidized Mn oxides until the oxidation level of the oxides was reduced to about 3.8, presumably reflecting the saturation by Mn2+ of highly reactive surface adsorption sites. Fully oxidized Mn oxides in sediments, then, may act as a cap preventing Mn2+ escape. We speculate that in shallow sediments of the Skagerrak, surface Mn oxides are present in a somewhat reduced oxidation level (< 3.8) allowing Mn2+ to escape, and perhaps providing the Mn2+ which enriches sediments of the deep basin.     

Molecular and isotopic analysis of anaerobic methane-oxidizing communities in marine sediments

Convergent lines of molecular, carbon-isotopic, and phylogenetic evidence have previously indicated (Hinrichs, K.-U., Hayes, J.M., Sylva, S.P., Brewer, P.G., DeLong, E.F., 1999. Methane-consuming archaebacteria in marine sediments. Nature 398, 802–805.) that archaea are involved in the anaerobic oxidation of methane in sediments from the Eel River Basin, offshore northern California. Now, further studies of those same sediments and of sediments from a methane seep in the Santa Barbara Basin have confirmed and extended those results. Mass spectrometric and chromatographic analyses of an authentic standard of sn-2-hydroxyarchaeol (hydroxylated at C-3 in the sn-2 phytanyl moiety) have confirmed our previous, tentative identification of this compound but shown that the previously examined product was the mono-TMS, rather than di-TMS, derivative. Further analyses of ¹³C-depleted lipids, appreciably more abundant in samples from the Santa Barbara Basin, have shown that the archaeal lipids are accompanied by two sets of products that are only slightly less depleted in ¹³C. These are additional glycerol ethers and fatty acids. The alkyl substituents in the ethers (mostly monoethers, with some diethers) are non-isoprenoidal. The carbon-number distributions and isotopic compositions of the alkyl substituents and of the fatty acids are similar, suggesting strongly that they are produced by the same organisms. Their structures, n-alkyl and methyl-branched n-alkyl, require a bacterial rather than archaeal source. The non-isoprenoidal glycerol ethers are novel constituents in marine sediments but have been previously reported in thermophilic, sulfate- and nitrate-reducing organisms which lie near the base of the rRNA-based phylogenetic tree. Based on previous observations that the anaerobic oxidation of methane involves a net transfer of electrons from methane to sulfate, it appears likely that the non-archaeal, ¹³C-depleted lipids are products of one or more previously unknown sulfate-reducing bacteria which grow syntrophically with the methane-utilizing archaea. Their products account for 50% of the fatty acids in the sample from the Santa Barbara Basin. At all methane-seep sites examined, the preservation of aquatic products is apparently enhanced because the methane-oxidizing consortium utilizes much of the sulfate that would otherwise be available for remineralization of materials from the water column.